<!DOCTYPE html>
<style>
div {
  width:300px;
  display:flex;
  justify-content:flex-end;
}
selectlist {
  width:100px;
}
selectlist::part(button) {
  border-style:none;
  background-color:rgba(0,0,0,0)
}
</style>
<div>
<selectlist>
  <option></option>
</selectlist>
